An SPL Logger Keeps You Consistent
SPL measurement can be a great tool! There are many options for software and some of the best for taking readings and collecting data is free.
If you want to start SPL logging on shows you might need some guidance as to where in the room to measure. The microphone reporting the level information is only reporting for that spot. If your system is designed correctly and your microphone strategically placed, you can interpolate the levels anywhere else in the room. For an explanation see this article: https://www.prosoundweb.com/up-periscope-using-a-probe-when-youre-not-in-the-room/
Once you know how the levels will translate over your space or you want to take readings at FOH or somewhere else, you will need to interpret the data. The real-time meter is somewhat helpful, mine shows the digital headroom left in DBFS, the weight of the SPL measurement, the speed of the averager, the type of SPL, an RMS max and RMS minimum and a peak value.
For the multi-day lectures I'm monitoring I use the "C" weighted SPL at a slow averager speed. Different scenarios lead to uses of other types of SPL measurement but for this example I'll show what I used. Any SPL measurement software will make you perform a calibration process. This matches received pressure levels with a certain gain setting. Room EQ Wizard has a "wizard" that will guide you thorough the process and it will require a microphone calibrating unit or a previously calibrated SPL meter. One note is that there are very stringent criteria for calibration if you are in certain jurisdictions, not all software and equipment is good enough for legally binding readings, this configuration is for data collection purposes only.
My default second screen arrangement is pictured below. At the top is the dual channel FFT which shows the response of the microphone over frequency in green and the response of the output of the console in red. I can easily monitor the difference and react to issues. In the bottom right is the real-time SPL window which includes all the above information. The logger is the lower left window.
Oftentimes you are looking to not exceed a limit which is arbitrary or that which is set by the venue or authorities. With a band this could be done with the main mix, the master fader or even a limiter or compressor on the main bus.
With corporate lectures I usually get the voice in the ballpark of my "max" and then apply a high ratio compressor to stop any peaks from exceeding the max. This can be achieved by using make up gain or not. This peak style compression works because people are constantly inhaling and the next few words are louder than when they are at the end of their breath cycle. The stiff ratio stops the extended peaks and the make up gain brings up the lower words.
To look at the instant meter, look at the compressor screen, adjust and then go back and forth is tiresome. This is where the logger enters as a good tool. Now you can see the past few seconds or minutes depending on your zoom level and it is easier to adjust your compressor so the real-time levels do not exceed your max and align with the past data.
In a macro view of the event, different presenters have different loudness levels to their voices. You may have to make sizable adjustments to get them to the same relative volume as the one before. With the logger this is easy.
Here is the same lecture zoomed to show 3 minutes instead of 3 hours. 70 dB SPL was my limit that I set and as you can see the peaks didn't surpass that.
Using a logger would also be good for theater shows, Church services, bands etc. Beware false readings though. If the mic is out there in a audience full of rowdy people it could get bumped or people near it could be yelling and it could throw off the readings. Keep an eye on the mic if possible.
A look at the even bigger picture would be to get repeatable results from show to show. I may end up in the same ballroom, with the same PA, with many of the same presenters for this one particular client so I will bring up the SPL log of the last time and get similar levels again. It's the ultimate consistency.
The nice feature about the logs is that unlike an audio recording, they are small files. 4 hours is about 5 MB. I have a database going and I back up all files to a cloud service.
Here is the link to the freeware that I use, it does many other things besides SPL measurement as well.
© 2019 Michael Reed